Most micro electro mechanical system (MEMS) microphones are designed as capacitive microphones where a thin
conductive membrane is located in front of a rigid counter electrode. The membrane is exposed to the environment to
convert sound into vibrations of the membrane. The movement of the membrane causes a change in the capacitance
between the membrane and the counter electrode. The resonance frequency of the membrane is designed to occur above
the acoustic spectrum to achieve a linear frequency response.
To obtain a good sensitivity the thickness of the membrane must be as small as possible, typically below 0.5 μm. These
fragile membranes may be damaged by rapid pressure changes. For cell phones, drop tests are among the most relevant
reliability tests. The extremely high acceleration during the drop impact leads to fast pressure changes in the microphone
which could result in a rupture of the membrane.
To overcome this problem a stable protection layer can be placed at a small distance to the membrane. The protective
layer has small holes to form a low pass filter for air pressure. The low pass filter reduces pressure changes at high
frequencies so that damage to the membrane by excitation in resonance will be prevented.
To miniaturize MEMS microphones we have developed a microphone package using flip chip technology instead of chip
and wire bonding. In this new packaging technology MEMS and ASIC are flip chip bonded on a ceramic substrate. The
package is sealed by a laminated polymer foil and by a metal layer. The sound port is on the bottom side in the ceramic
substrate. In this paper the packaging technology is explained in detail and results of electro-acoustic characterization
and reliability testing are presented. We will also explain the way which has led us from the packaging of Surface
Acoustic Wave (SAW) components to the packaging of MEMS microphones.